This invention relates to power amplifiers for controlling the current flow through a load, such as a d-c servo motor. More particularly, the invention relates to power amplifiers of the type comprising a plurality of switching elements connected in bridge fashion with the load and a predetermined power source for coupling the load across alternate output terminals of the power source in accordance with the levels of a plurality of control signals respectively applied to the plurality of switching elements.
Switching power amplifiers of the bridge type normally include diagonally opposed power switching elements which are caused to conduct current through the load (e.g., motor) in a given direction, with one set of diagonally opposed elements causing motor current flow in a forward direction, and the remaining set of diagonally opposed elements allowing motor motor current flow in the reverse direction. By controlling the conducting intervals of the sets of diagonally opposed switching elements, current flow through the motor can be proportionally controlled. Exemplary bridge type switching power amplifiers employing pulse width modulation (PWM) techniques are disclosed in U.S. Pat. Nos. 3,422,326, 3,471,759 and 3,525,029, and an exemplary so-called "free-running" bridge type switching power amplifier is disclosed in now abandoned U.S. application No. 684,522 filed on May 10, 1976 in the names of Frank D. Ruble et al for CONSTANT CURRENT POWER AMPLIFIER and assigned to the assignee of the present invention.
The plurality of control signals for respectively controlling the states of the plurality of switching elements are normally generated by circuitry which is connected to a predetermined source of power. Oftentimes, a "glitch" or unexpected loss of power may occur which may alter the levels of the control signals and thus the states of the switching elements. This may, in turn, lead to imprecise load control, and possibly even damage to the load.
It would be desirable, therefore, to provide in a switching power amplifier of the bridge type a means of detecting a glitch in the power supply and for inhibiting the current flow through the load in response to a detected glitch. It would further be desirable if the current flow were inhibited only if and when the glitch, or power supply reduction, was the result of a voltage reduction below a predetermined reference potential for at least a first predetermined time duration, so as to avoid unnecessary shut downs caused by electrostatic noise and the like.
It would also be desirable if, once the glitch in the power supply ceases by the power supply voltage returning to a desired level, resumed controlled current flow through the load is delayed for at least a second predetermined time duration. This will insure that the overall system of which the power amplifier is a part is allowed to stabilize following cessation of the glitch prior to resumption of controlled current flow through the load. This is especially important where the monitored power supply also is coupled to other portions of the system for supplying power thereto.
It would further be desirable if the power amplifier could inhibit initial controlled current flow through the load for a third predetermined time interval following initial power-on of the system, thereby further enabling the system to fully stabilize prior to initial controlled current flow through the load.